Hello I am new here and am hoping someone here can help me with this problem. I have a 95 f150 with a 302 and duel fuel tanks. My rear tank keeps building up pressure to the point when you open the cap it blows out fuel at me. I can blow through the vent lines. The canister is not plugged and the purge solenoid is working fine. However if I disconnect the vent line from the canister no pressure releases unless I open the cap. It does not matter which tank I run the rear will build up pressure. I replaced both tanks about a year ago but this problem just started about 2 months ago. Does any one know what is going on. I've never heard of a tank building pressure.
Please help me

 

It's a common problem with the shuttle valve inside the FDM. It sticks, allowing fuel to be transferred in from the OTHER tank. So the bad one is the one that's RECEIVING fuel. Click the black Bronco in my signature & look in the EFI Fuel Pump album. There are diagrams that show how it all works. When you drop the tank(s), pull the rollover valve & make sure it flows air when upright, but not when inverted.

 

I'll Drop the tanks tomorrow. This problem has been driving me nuts. Thanks so much.

 

How do I get the shuttle valve off of the sending unit without breaking it?

 

You don't. You buy a whole new FDM either from your local dealer ($$$$$) or from somewhere like rockauto.com ($$).


...and save the old pump & level sender. The pump motor is probably OK, and I don't think the level sender isn't available separately, so it's good to have a spare.

 

Is there a way to just stop it from pressurizing and just run the front tank. Like maybe just leave it unplugged?

 

Not without damaging the fuel lines. This valve isn't electrical. Have you looked at the diagram in that album?

 

How will it damage fuel lines? So far it seems to be working. And my gas mileage has improved greatly. I understand that its not electrical but I figured it was worth a shot. I don't know maybe its not that I left it unplugged, its just the fact that I took everything a part and then put it back together. I guess only time will tell. Thank You for your help. It would have been a long time before I figured that out.

 

Unplugging it won't damage the fuel lines - stopping the pressure from reaching the leaking tank would involve damaging the lines, because you'd have to crimp them.

Unless you took the FDM apart, you didn't do anything to the sticking valve. The only thing I can imagine helping would be a can or 2 of pour-in Berryman's B-12 Chemtool in the tank that RECEIVES fuel.

 

So far the only thing I've done is left the harness unplugged. Aside from taking the thing as apart as I could get it. I did move the valve back a forth a few times, but it still stopped me from blowing through it no matter which way I tipped it unless I manually oped the valve. I don't understand how leaving unplugged does anything either. I guess I could plug it back in and see if the pressure comes back. But I'm the kind of person where if it isn't broken or bothering me I leave it alone. Although my wife did run it out of fuel two days later. But I don't believe thats because it was sending fuel to the rear tank I think thats because shes a woman.

 

What valve are you talking about? You can't touch the shuttle valve without destroying the FDM, which is inside the tank.

 

This valve was part of the sending unit right next to where the fuel pump attaches. from the diagrams you gave me I interpreted it as the shuttle valve. The actual valve was inside of the sending unit but it had a little plunger with son sort of washer/clip on the outside that I was able to grab. As it sat I was unable to blow through the sending unit but if I grabbed the plunger and opened it I was able to blow through it. I thought this was what You told be to do. I haven't yet had the time to reconnect the harness but still no pressure.

 

Do you mean this?The rollover valve isn't part of the FDM - it's in the top of the tank. That test makes sure the tank LOSING fuel is venting properly.